Contamination control, including gas-borne and liquid-borne particulate monitoring, plays a critical role in the manufacturing processes of several industries. These industries require cleanrooms or clean zones with active air filtration and require the supply of clean raw materials such as process gases, de-ionized water, chemicals, and substrates. In the pharmaceutical industry, the Food and Drug Administration requires particulate monitoring because of the correlation between detected particles in an aseptic environment and viable particles that contaminate the product produced. Semiconductor fabrication companies require particulate monitoring as an active part of quality control. As integrated circuits become more compact, line widths decrease and wafer sizes increase such that the sizes of particulates causing quality problems become smaller. Detection of particles in liquids, especially in water, is also of great importance for environmental protection.
To perform particulate monitoring, currently available commercial submicron particle sensors use direct optical detection techniques to determine the presence, size, and/or number of particles. The detection and discrimination of 0.08 .mu.m particles is the highest sensitivity currently achievable by direct optical detection techniques for airborne particle counting and/or sizing at a typical sample flow rate of 1.0 cubic foot per minute. This sensitivity is achieved by employing optical scattering from He--Ne laser intracavity light. The 0.08 mm particle detection size limit for gas-borne particle detection stems from strong background Rayleigh (molecular) scattering in gases. The detection and discrimination of 0.05 .mu.m particles is the highest sensitivity currently achievable by direct optical detection techniques for liquid-borne particle counting and/or sizing at a typical sample flow rate of 4 milliliters/minute. The 0.05 .mu.m particle detection size limit for liquid-particle detection stems from strong Brillouin scattering in water. One manufacturer, Particle Measuring Systems, Inc., Boulder, Colo., uses in its Micro LPC-H airborne particle detector and in its Micro DI50 liquid particle detector a direct optical detection technique to produce these particle sensitivities. These latter sensitivities are state of the art for commercial particle detectors, but they are not recent developments.
The basic building block for this technology is optical scattering of laser light and direct detection of the optical signal scattered by the particles. Because the optical signal scattered by submicron particles has a small scattering cross-section, high incident optical intensities are necessary to achieve detectability. In the past for gas-borne particle detectors, the economical and practical approach to achieving the required optical intensity levels (on the order of 10,000 watts/cm.sup.2) was to use a multilongitudinal mode and multilateral mode He--Ne laser. Such lasers exhibit, however, significant amounts of noise, some of which is cancellable by complicated noise cancellation schemes and some of which is not cancellable. Noise cancellation schemes typically entail the use of extra optical detectors to monitor the fluctuations of the intracavity optical intensity so that their effects might be removed from the weak particle signal. Moreover, multimode lasers are implemented with brute force direct optical detection. For liquid-borne particle detectors, the economical and practical approach to achieving the required optical intensity levels was to use a focused laser (typically diode laser) beam, thereby increasing local optical intensity while reducing the background scattering by reducing the size of the view volume containing the particle. The reduction of the view volume had, however, the undesirable effect of simultaneously reducing the sample rate.
Although these cumbersome methods have had significant success leading to the detectability levels described above, there is a fundamental inherent limit to the direct detection method that has prevented any recent significant advance in particle detection sensitivity. Background scattering limits the minimum particle size sensitivity achievable by lasers using direct optical detection techniques. The standard approach, which was developed during the late 1980's, to overcoming shot noise associated with the background light was to minimize the view volume a given detector sees. For gas-borne detection this approach was implemented by division of the total sample view volume into as many as possible subvolumes by the use of arrays of photodiode detectors, each photodiode detector viewing only one subvolume and thus a smaller amount of background noise. For liquid-borne particle detectors the view volume was decreased using a focused laser beam as described above. This became and has since remained the state of the art.
Several drawbacks of the He--Ne laser limit its performance as a tool for optical detection of particles. Of the several drawbacks, three are immediately obvious. First, the He--Ne laser has an intrinsically low gain and, therefore, is susceptible to minor intracavity losses such as contamination from the ambient environment it is meant to sample. Second, the He--Ne laser employs high voltages, typically on the order of 2 kV. Third, the electrically charged window on the He--Ne laser tube attracts contamination in a manner similar to the way a computer monitor or television screen does.
As mentioned above, current optical scattering particle counters use photodiodes to implement direct optical detection techniques. For such direct optical detection, the signal-to-noise power ratio at the amplifier output is given by (A. Yariv, Optical Electronics, Holt, Reinhart, and Winston, 1985, 3rd Edition) ##EQU1## where P is the optical signal power resulting from scattering by a particle of interest, e is the electronic charge, .eta. is the detector quantum efficiency (i.e., the average number of carriers generated for an incident photon), h is Planck's constant, .nu. is the optical carrier frequency, P.sub.B is the background optical signal power, i.sub.d is the diode photodetector dark current, .DELTA..nu. is the frequency bandwidth of the detection electronics, k is Boltzmann's constant, and T.sub.e is the effective temperature chosen so that the last term in the denominator accounts for the thermal noise of the load, R.sub.L, across the detector as well as for the noise generated by the amplifier that follows the detector. The denominator in equation (1) contains four terms, each representing an independent noise source. The first term represents signal shot noise, which cannot be eliminated. The second term is caused by background radiation shot noise. The third term is the result of detector dark current, and the fourth term represents noise caused by the load resistance, R.sub.L, across the output of the photodiode and the amplifier following the detector.
It is well known that the signal-to-noise ratio and thus the lower limit on the particle size for gas-borne particle counters is limited by background noise generated by Rayleigh scattering from the gas carrier in which the particle is immersed. The signal-to-noise ratio and thus the lower limit on the particle size for liquid-borne particle counters is about the same as that of gas-borne particle counters but results from strong Brillouin scattering in liquids such as water.
This situation is called "background shot noise limited" because the shot, or quantum, noise associated with the background radiation is the major source of noise in the optical detection system. An ideal detection system would be one in which the particle size detection sensitivity is at the signal quantum noise detection limit. The practical achievement of this limit using direct optical detection is infeasible because it would depend on total suppression of other noise sources, such as detector dark current, in addition to the background signal.